Fig. 4. Vascular development in wild type and ral1. (A-C,G,I,K,O-R,W,Y) Wild type. (D-F,H,J,L-N,S-V,X,Z) ral1. (A-F) Transverse sections through 2-week-old seedlings, 10 µm above the insertion of the first (A,D), second (B,E) and third (C,F) leaf primordium on the shoot apex (sixth, fifth and fourth leaf, respectively). (G-J) DR5-GUS (G,H) or Oshox1-GUS (I,J) expression in transverse vibratome sections (100 µm) through the shoot apex of 2-week-old seedlings. (K-N) Transverse section 200 µm above the shoot apex of 2-week-old seedlings showing Oshox1-GUS expression that identifies commissural veins developing in the fourth leaf primordium (third leaf). (O-V) DR5-GUS (O,P,S,T) or Oshox1-GUS (Q,R,U,V) expression in vascular bundles at a comparable stage of differentiation in the fourth (second protoxylem element stage; O,S,Q,U) and fifth (late metaxylem element stage; P,T,R,V) leaf primordium (third and second leaf, respectively). Xylem is oriented to the right. (W,X) Oshox1-GUS expression in mature embryos (dorsal view). (Y,Z) Schematic representation of the dorsal view of mature embryos showing the vascular system of the scutellum expressing (in blue) or not (in black) Oshox1-GUS. 1, 2, 3, first, second, and third leaf primordium, respectively; pp, protophloem; pv, provascular strand; px, protoxylem. Scale bars: (A-F) 100 µm (G-N) 50 µm (O-V) 25 µm (W,X) 350 µm.